1. Field of the Invention
This invention relates to a silver halide photosensitive material containing at least one kind of silver halide emulsion which has enhanced light absorption, a method of preparing the same, and a silver halide photosensitive material containing the same, especially silver halide photosensitive material having a greatly improved spectral sensitivity resulting from an enhancement in light absorption, and also having improved preservability.
2. Description of the Related Arts
A great deal of effort has hitherto been made for increasing the speed of a silver halide photosensitive material. In silver halide emulsions, the light entering a photosensitive material is absorbed by silver halide grains or by sensitizing dyes adsorbed on the surfaces of the grains and then the light energy is transmitted to the silver halide grains. Thus, light sensitivity is achieved.
Silver halide grains can absorb only light of a wavelength shorter than the blue region. In order to make silver halide grains sense the light having a wavelength longer than that region, a technique of making sensitizing dyes adsorbed on the surfaces of silver halide grains absorb the light and transmitting the light energy to the grains is used. This so-called “spectrally sensitizing technique” has become an essential and important technique for enhancing the sensitivity of silver halide photosensitive materials. Tabular silver halide, which is widely used nowadays, has a high light permeability and a low absorptivity even in its intrinsic absorbing region. Therefore, spectral sensitization has an important role even in the intrinsic region.
The light absorptivity achieved by spectrally sensitizing dyes on the surfaces of silver halide grains can be improved by increasing the amount of dyes adsorbed per unit grain surface area by use of dyes having a high molecular extinction coefficient.
For this purpose, many photographic systems have employed so-called J-aggregating cyanine dyes, which have a high molecular extinction coefficient and which are believed to adsorb to and pack together on the dye molecules' “edge” surface which allows the maximum number of dye molecules to be placed on the grain surfaces.
Tabular silver halide has also been used as a measure for increasing the amount of sensitizing dyes which can adsorb. Increasing the surface area of grains by making the grains have a tabular form and allowing the aforementioned J-aggregating cyanine dyes to adsorb onto the surfaces of the grains can increase the light absorptivity. Therefore, tabular grains have been used in many photographic systems.
However, despite such efforts, there are limitations to the amount of sensitizing dyes which can adsorb to the surface of a silver halide grain. It is impossible to allow dyes to adsorb in an amount more than that achieved by monolayer saturation adsorption (i.e., monolayer adsorption). Therefore, in most photographic systems, it is still the case that not all the available light is being collected.
Described below are methods proposed as methods by which a greater light absorption can be achieved for solving that problem.
One method is an approach to make a plurality kinds of dyes adsorb onto silver halide grains by multi-layered adsorption, thereby causing Forster type excitation energy transfer. This was proposed by G. R. Bird in Photographic Science and Engineering, Vol. 18, page 562 (1974). Another approach proposed in U.S. Pat. Nos. 2,518,731, 3,622,316, 3,976,493 and 3,976,640 and Japanese Patent Application KOKAI Publication No. (hereinafter referred to as JP-A-) 63-138341 is to use dyes in which two dye chromophores are covalently connected. As a method for improve the Bird's proposal, Ukai et al. proposed in JP-A-64-91134 a method in which at least one substantially non-adsorbing dye containing at least two sulfo and/or carboxyl groups selected from cyanine dyes, merocyanine dyes and hemicyanine dyes is linked to an adsorbing dye. This method, however, is defective from the facts that when two kinds of dyes are linked, their performances are, contrary to expectations, disturbed mutually so that they cannot aggregate or adsorb properly on silver halide grains and therefore only an enhancement in sensitivity less than expected is obtained; a great increase in dye molecule volume caused by the linking reflects dye stain after development (unfavorable increase in D-min caused by a sensitizing dye remaining after development); and complication of production reflects a great increase in the cost of the linked dye, which also increases the production cost of photosensitive materials.
As a method using linked dyes, L. C. Vishwakarma, JP-A-6-57235 propose a method of using a dye in which a monomethine cyanine dye and a pentamethine oxonol dye are covalently connected. However, in this case, spectral sensitization due to Forster type excitation energy transfer between dyes was not effected because the luminescence of the oxonol dye did not overlap with the absorption of the cyanine dye. Therefore, higher sensitization by the light converging function of the connected oxonol dye cannot be aimed.
Another approach is a method in which, contrary to the aforementioned methods, two dye chromophores are not connected.
As this approach, Sugimoto et al. disclose in JP-A's-63-138341 and 64-84244 spectral sensitization utilizing energy transfer from luminescent dyes using an emulsion spectrally sensitized with an adsorbing sensitizing dye used in combination with a non-adsorbing luminescent dye which is located in gelatin. As a similar approach, R. Steiger et al. propose, in U.S. Patent No. (hereinafter referred to as USP) U.S. Pat. No. 4,040,825 and U.S. Pat. No. 4,138,551 and R. Steiger, Photographic Science and Engineering, Vol. 27(2), page 59 (1983), silver halide photosensitive material spectrally sensitized with an adsorbing sensitizing dye used in combination with a second dye which is bonded to gelatin.
However, the problem with this approach is that although the dyes are less likely to interfere with one another, efficient energy transfer will not occur unless the dye not adsorbed to the grain is in close proximity to the dye adsorbed on the grain (less than 50 angstroms separation) (see T. Forster, Annalen der Physiks, Vol. 6 (2), page 55 (1948) and T. Forster, Discussions Faraday Society., Vol. 27, page 7 (1959)). Most non-adsorbing dye will not be close enough to the silver halide grain for energy transfer and, therefore, will act only as a filter dye leading to a speed loss. An analysis of this problem is given by Steiger et al. (see R. Steiger, Photographic Science and Engineering, Vol. 27(2), page 59 (1983)).
A better method proposed is to form two or more dye layers on silver halide grains. P. B. Gilman discloses, in P. B. Gilman, Photographic Science and Engineering, Vol. 20(3), page 97 (1976), a method in which a cation dye is made adsorb as a first layer which adsorbs directly to a silver halide grain and an anion dye which can absorb the light of a wavelength longer than the dye of the first layer is used. However, this method cannot cause the Forster type efficient excitation energy transfer from the dyes of the second layer to the dyes of the first layer.
G. R. Bird et al. propose in JP-A-64-91134 a similar system which is so designed that the dyes in the second or outer layer (namely, the outer dye layer) can absorb the light of a wavelength shorter than the dye of the first layer (namely, the inner dye layer which is adsorbed directly to the silver halide grain). However, this proposed method makes color reproduction very poor because it forms a sensitization region of an extremely wide wavelength range and therefore the same silver halide grain senses both green light and red light.
Yamashita et al. also disclose a method in which two or more kinds of cyanine dyes are used and multi-dye layers satisfying the necessary condition provided by G. R. Bird et al. are formed on a silver halide grain. They disclose in JP-A's-10-123650 and 10-239789 that the necessary condition is to use a dye having an aromatic group and that a preferred necessary condition is to use a cation and anion dyes both having an aromatic group. However, this approach is disadvantageous in that to make the dyes have aromatic groups can lead to large amounts of retained dye after processing, which affords greatly increased D-min. In silver halide color photosensitive material, a silver halide-containing layer is made to contain an emulsion including an emulsified and dispersed coupler capable of forming color by being coupled with an oxidized developing agent during development. However, the method is disadvantageous in that the multi-layeredly adsorbed layers are damaged if that emulsion is made to exist together with them.
Yamashita et al. disclose, as an approach which can improve the above-mentioned disadvantage, a method in which a cation and anion dyes having poly-electric charges are combined to form multi-dye layers (see JP-A-10-171058). A concept and a method the same as those mentioned above are disclosed by R. L. Parton et al. in JP-A's-2000-89405 and 2001-117192. It is also disclosed in JP-A-2001-117191 that a preferable multi-dye layers can be formed when at least one of the dyes is further substituted with a hydrogen bonding donor substituent.
These approaches are superior, as approaches for forming multi-dye layers, to those heretofore proposed, but are not yet satisfactory. In other words, despite some improvement accomplished, it is still the case that when a color coupler emulsion is made exist, the multiple adsorbed layers formed cannot be maintained, leading to decrease in dye adsorption amount.
That is, the interaction between the outer layer dye that is not adsorbed directly to the silver halide grain and the inner layer dye that is adsorbed directly to the grain is still weak, and therefore desorption of the outer layer dye arises when the emulsion is left to stand for a long time in a dissolved state or the photosensitive material is store under a high temperature and a high humidity. This leads to decrease in absorbing intensity together with decrement in speed. Especially in a silver halide color photosensitive material, various water-insoluble photographically useful compounds are dissolved in a high boiling organic solvent, and added in a form of an aqueous dispersion in which the organic solvent is emulsified. It is well known for the relevant technicians that the sensitizing dye that absorbs to silver halide grains is apt to be desorbed by the dispersion. Various improvements have been done for suppressing the desorption. However, even with such improving techniques, the interaction between the inner layer dye and the outer layer dye in the multi-layered dye layer is still weak compare with that between sensitizing dyes that absorb directly to silver halide grains, as mentioned above. Accordingly, significant desorption of the outer layer dyes arises when the emulsion to which the aqueous dispersion is added is left to stand for a long time in a dissolved state or the photosensitive material is stored in a high temperature and a high humidity conditions. As a result, even if the absorbing intensity and speed are greatly increased by forming the outer dye layer, actually, almost all of it are lost. In the course of preparation, it is necessary for an emulsion in which sensitizing dyes are adsorbed to store in a dissolved state before coating the emulsion.
On the contrary, a technique of using an aqueous dispersion of water-insoluble photographically useful compounds such as couplers which was prepared by using a specific surfactant and a specific high boiling organic solvent, is disclosed in for example JP-A-2002-148767. Although such improving technique attain a certain improvement, it is not still unsatisfactory for applying it to a practical production.